1. Field of the Invention
The present invention relates to techniques for determining downhole parameters in a wellbore and/or surrounding formation.
2. Background of the Related Art
Wellbores are drilled to locate and produce hydrocarbons. A string of downhole pipes and tools with a drill bit at an end thereof, commonly known in the art as a drill string, is advanced into the ground to form a wellbore penetrating (or targeted to penetrate) a subsurface formation of interest. As the drill string is advanced, a drilling mud is pumped down through the drill string and out the drill bit to cool the drill bit and carry away cuttings and to control downhole pressure. The drilling mud exiting the drill bit flows back up to the surface via the annulus formed between the drill string and the wellbore wall, and is filtered in a surface pit for recirculation through the drill string. The drilling mud is also used to form a mudcake to line the wellbore.
It is often desirable to perform various evaluations of the formations penetrated by the wellbore during drilling operations, such as during periods when actual drilling has temporarily stopped. In some cases, the drill string may be provided with one or more drilling tools to test and/or sample the surrounding formation. In other cases, the drill string may be removed from the wellbore (called a “trip”) and a wireline tool may be deployed into the wellbore to test and/or sample the formation. Various drilling tools and wireline tools, as well as other wellbore tools conveyed on coiled tubing, are also referred to herein simply as “downhole tools.” The samples or tests performed by such downhole tools may be used, for example, to locate valuable hydrocarbons and manage the production thereof.
Formation evaluation often requires that fluid from the formation be drawn into a downhole tool for testing and/or sampling. Various devices, such as probes and/or packers, are extended from the downhole tool to isolate a region of the wellbore wall, and thereby establish fluid communication with the formation surrounding the wellbore. Fluid may then be drawn into the downhole tool using the probe and/or packer.
A typical probe employs a body that is extendable from the downhole tool and carries a packer at an outer end thereof for positioning against a sidewall of the wellbore. Such packers are typically configured with one relatively large element that can be deformed easily to contact the uneven wellbore wall (in the case of open hole evaluation), yet retain strength and sufficient integrity to withstand the anticipated differential pressures. These packers may be set in open holes or cased holes. They may be run into the wellbore on various downhole tools.
Another device used to form a seal with the wellbore sidewall is referred to as a dual packer. With a dual packer, two elastomeric rings are radially expanded about a downhole tool to isolate a portion of the wellbore wall therebetween. The rings form a seal with the wellbore wall and permit fluid to be drawn into the downhole tool via the isolated portion of the wellbore.
The mudcake lining the wellbore is often useful in assisting the probe and/or dual packers in making the appropriate seal with the wellbore wall. Once the seal is made, fluid from the formation is drawn into the downhole tool through an inlet therein by lowering the pressure in the downhole tool. Examples of probes and/or packers used in downhole tools are described in U.S. Pat. Nos. 6,301,959; 4,860,581; 4,936,139; 6,585,045; 6,609,568 and 6,719,049 and U.S. Patent Application No. 2004/0000433. Such devices may be used to perform various sampling and/or testing operations. Examples of so-called ‘pretest’ techniques used in some such operations are described for example in U.S. Pat. Nos. 6,832,515, 5,095,745 and 5,233,866.
In some cases, it is necessary to penetrate the sidewall of the wellbore and casing and cement (if present). Techniques have been developed to create holes or perforations through the sidewall and reach the surrounding formation. Examples of such techniques are described in U.S. Pat. No. 5,692,565. It is sometimes desirable to close the holes created in the wellbore wall to prevent fluids from flowing into the wellbore. Examples of techniques that use plugs to fill such perforations are described in U.S. Pat. Nos. 6,426,917, 2,821,323, 3,451,583, 4,113,006, 4,867,333, 5,160,226 and 5,779,085. Techniques have also been developed to provide such plugs with sensors to measure downhole parameters as described, for example, in U.S. Pat. No. 6,766,854.
Despite such advances in downhole perforation and plugging, there remains a need for techniques capable of monitoring downhole parameters and/or plugging perforations in a wellbore wall. It is desirable that such a technique utilize a plug insertable into a wellbore wall and having circuitry capable of collecting data and/or communicating information. It is further desirable that such a plug be provided with one or more of the following, among others: a container to protect electronics from the harsh wellbore environment, a plug sleeve adapted to fit snugly in the perforation, electronics packaging positionable in the plug sleeve, operability in a variety of wellbore conditions (such as low permeability formations) and various downhole testing capabilities, such as a pretest.